1. Field of the Invention
The present invention relates to a device for the application of liquid or viscous coating medium onto a moving application surface by way of an applicator.
2. Description of the Related Art
When coating a material web by way of a curtain coater (also known in the field as “Curtain Coating”) the coating medium is supplied to the application surface in form of a coating medium curtain whose movement from the applicator to the application surface is essentially contingent upon gravity. The fact that the curtain coater is located at a certain distance from the application surface has the added advantage that, for example in the event of a web break, it has a lowered risk of damage exposure. Curain types of coaters differentiate from other types of “contact-less” applicator devices, for example open jet nozzle type coaters in which the movement of the coating medium from the applicator to the application surface stems primarily from the output impulse from the dispensing nozzle of the applicator, basically because the form of the curtain emerging from the dispensing nozzle is subjected only to the interplay between the surface tension of the coating medium and gravity. In this situation the surface tension attempts to pull together the curtain that, relative to its volume and its cross sectional surface possesses a very large surface or circumferential length, in order to thereby reduce its surface area. This effect is countered only by gravity which attempts to stretch the curtain. It is therefore easily understood that it is more and more difficult with increasing working widths to obtain a coating medium curtain that is uniformly thick across the entire working width.
Coating of material webs by way of curtain coaters that supply the coating medium to the material web in the form of a coating medium curtain or veil whose movement is essentially based upon gravity has been known for some time from coating of photographic films, audio tapes, etc. The material webs associated with this type of application are much narrower than those in a modern line for the production of paper and cardboard webs where web widths of more than 10 m are required. To be able to form and hold stable a uniform coating medium curtain across such widths is a task for which suggestions for functional solutions cannot readily be found in the comparatively easily controlled, known narrow coating medium curtain applications. In addition, material webs in modern lines for the production of paper and cardboard webs run at speeds of up to 3000 m/min. This is many times faster than the speeds with which the known narrow material webs run and represents an additional high demand on the stability of the coating medium curtain.
DE 199 03 559 A1 describes several working principles which are intended to weaken the boundary layer of air that is carried along by the material web, immediately prior to the curtain coater. However, DE 199 03 559 A1 does not address feasibilities of improving the efficiency of these working principles. A multitude of elements are known from WO 01/16427. These are positioned against the material web surface, immediately prior to the curtain coater when viewed in direction of material web travel, in order to prevent the boundary layer of air causing interference with the coating medium curtain. For the sake of completeness with regard to additional state of the art we also refer to DE 197 16 647 A1, DE 199 03 559, DE 198 03 240 A1, DE 198 294 49 A1, EP 0974403 A1, as well as the priority older, but later published applications DE 100 12 347 and DE 100 57 734.
The use of a blowing device alone and the use of a suction device alone for facilitating the weakening of the boundary layer of air is basically known from the main application DE 100 12 257. However, in both scenarios strips, brushes or doctor elements are also utilized. They either weaken the boundary layer of air prior to entry into the working range of the suction device, or intend to cause further weakening of the remaining boundary layer of air that is already pre-weakened by the blowing device. These components which are in contact with the material web cause undesirable wear and tear on the material web, as well as on the weakening device. Consequently, this involves the risk of a web break and also increased maintenance costs, as well as increased spare parts expenditure.